Polyolefins have limited use in engineering applications due to the tradeoff between toughness and stiffness. For example, polyethylene is widely regarded as being relatively tough, but low in stiffness. Polypropylene generally displays the opposite trend, i.e., is relatively stiff, but low in toughness.
Several well known polypropylene compositions have been introduced which address toughness. For example, it is known to increase the toughness of polypropylene by adding rubber particles, either in-reactor resulting in impact copolymers, or through post-reactor blending. However, while toughness is improved, the stiffness is considerably reduced using this approach.
Glass reinforced polypropylene compositions have been introduced to improve stiffness. However, the glass fibers have a tendency to break in typical injection molding equipment, resulting in reduced toughness and stiffness. In addition, glass reinforced products have a tendency to warp after injection molding.
Another known method of improving physical properties of polyolefins is synthetic organic fiber reinforcement. For example, EP Patent Application 0397881, the entire disclosure of which is hereby incorporated herein by reference, discloses a composition produced by melt-mixing 100 parts by weight of a polypropylene resin and 10 to 100 parts by weight of polyester fibers having a fiber diameter of 1 to 10 deniers, a fiber length of 0.5 to 50 mm and a fiber strength of 5 to 13 g/d, and then molding the resulting mixture. Also, U.S. Pat. No. 3,639,424 to Gray, Jr. et al., the entire disclosure of which is hereby incorporated herein by reference, discloses a composition including a polymer, such as polypropylene, and uniformly dispersed therein at least about 10% by weight of the composition staple length fiber, the fiber being of man-made polymers, such as poly(ethylene terephthalate) or poly(1,4-cyclohexylenedimethylene terephthalate).
Fiber reinforced polypropylene compositions are also disclosed in PCT Publication WO02/053629, the entire disclosure of which is hereby incorporated herein by reference. More specifically, WO02/053629 discloses a polymeric compound, comprising a thermoplastic matrix having a high flow during melt processing and polymeric fibers having lengths of from 0.1 mm to 50 mm. The polymeric compound comprises between 0.5 wt % and 10 wt % of a lubricant.
Consistently feeding pre-cut synthetic organic fibers into a compounding extruder is an issue encountered during the production of fiber reinforced polypropylene composites. Pre-cut synthetic organic fiber, due to its low bulk density and tendency to entangle, is difficult to feed consistently into polymer compounding equipment. Gravimetric or volumetric type screw or auger feeders are used in the metering and conveying of polymers, fillers and additives into the extrusion compounding process. These feeders are designed to convey materials at a constant rate using a single or twin screw type of auger mechanism by measuring the weight loss in the hopper of the feeder or the volume of additive fed to the compounding extruder. These feeders are effective in conveying pellets or powder, but are generally not effective in conveying cut synthetic organic fiber. Two issues are generally encountered with traditional gravimetric or volumetric additive feeders when feeding cut organic fiber.
The first issue is that the cut fiber tends to bridge in the feed throat leading from the feeder hopper to the feeder metering auger or screw. This results in a non-uniform rate of fiber feeding the feeder screw or auger, which necessarily results in an inconsistent fiber feed rate to the compounding process. More particularly, at certain times, fiber gets hung up in the feeder throat area and little fiber is conveyed by the feeder, while at other times, an overabundance of fiber is conveyed to the compounding extruder. FIG. 1 is an illustrative plot of the feed rate of ¼ inch chopped polyester fiber through a typical single screw type gravimetric feeder (prior art). The feed rate may vary anywhere from 3 to 18 grams per 5 seconds of feeding. This inconsistency is less than adequate to produce a fiber reinforced polypropylene in an extruder with a consistent percentage of fiber incorporated into the polypropylene based resin.
A second issue encountered with typical gravimetric or volumetric additive feeders is that the pre-cut fiber has a tendency to clump at the end of the screw type auger of the feeder resulting in the fiber dropping in large clumps into the compounding extruder. These large pre-cut fiber clumps result in fiber feed rate inconsistency to the compounding extruder. This makes dispersion of the organic fiber into the polypropylene matrix more difficult because of the greater work the compounding extruder must do to uniformly disperse the organic fiber. It may also lead to variations in the fiber loading in the polypropylene composite as a function of time, which correspondingly may result in a variation in properties of the resultant articles molded from the composite pellets.
A need exists for improved form of synthetic organic fiber to more easily facilitate the feeding of the fiber into a compounding extruder for making fiber reinforced polypropylene composites. More particularly, a need exists for improved methods of feeding synthetic organic fiber into the polypropylene based resin during the compounding process while still maintaining the advantageous effects of the fiber on impact resistance and flexural modulus of parts molded from the composite resin pellets.